Fluid mixtures with two or more phases are handled in fluid systems in a wide range of industries. Such mixtures can include emulsions, solutions and particle suspensions or the like and may be moved or circulated through various flow spaces or conduits in such systems. Dependent on conditions and phase types, the mixtures may display a separation of phases and a tendency of one of the phases settling within another phase. This may be seen particularly if the fluids are at rest, poorly mixed or moving slowly in conduits. It can be important to handle the mixtures in such a way as to restrict or avoid settling. For example, it may be desirable to avoid a build up of residues or blockages in conduits. This may require knowledge of the behaviour of the fluid mixture with regard to settling, for example to allow appropriate constituents of the fluid to be selected. For example, in a mixture where a first insoluble phase constitutes particles within a fluid second phase, an appropriate amount or type of particles and fluid may be selected.
The term “settling” includes settling by sinking or rising of one phase within another phase.
The issue of settling is a particular concern in wellbore drilling operations, for example in the oil and gas industry, where drilling fluid is circulated through a well bore. The drilling fluid is pumped from a surface rig into the well through a drill string provided with a drill bit at its lowermost end for penetrating the subsurface rock formations. The fluid exits the drill string near the drill bit, and passes back up through the well to the surface rig in an annular space between an outer surface of the drill string and a wall of the wellbore.
The drilling fluid acts to lubricate the drill string, cool the drill bit and transport drill cuttings out of the well bore. In addition, it may be needed to seal permeable formations traversed by the wellbore. A sufficient hydrostatic pressure is also required in the well in particular to prevent gas, water or oil leaking into the well. The drilling fluid needs to have an appropriate composition in order to fulfil these functions.
In particular, in order to provide the hydrostatic pressure required, it is usual to increase the density of the drilling fluid, whether it be water or oil based, by adding insoluble particles to serve as weighting agents. Typically, barite particles are used.
Performance of a drilling operation depends on the particles staying suspended within the drilling fluid. This is usually achieved whilst circulating drilling fluid through the well. However, it is unavoidable that circulation is halted from time to time during a drilling operation. At such times, the insoluble particles will tend to settle in the fluid, out of suspension. For example, the drilling fluid may exhibit particle “sag” where settling particles fail to follow the circulation of the drilling fluid out of the wellbore. This can result in fluctuations in density of the drilling fluid exiting the well bore during an operation. Sag can be thought of as a settling of solid materials in a fluid owing to gravitational effects. Barite sag is described as the segregation of weighting material due to gravitational effects which can lead to density variations in drilling fluids. The problem can also occur with other high density weighting agents used to increase density, such as hematite. Barite sag is almost always characterized by drilling fluids having a density below nominal (surface) mud weight, followed by density above nominal when circulating bottoms-up after a trip.
There are various different mechanisms for sag, any of which can be in operation. These can be summarized as:                Static settling, owing to gravitational effects;        Dynamic settling, most common during slow pumping or movement of the fluid, and more severe than static sag; and        Slumping, owing to the Boycott effect, in which an inclined tube containing particles in a suspending fluid sets up an internal flow that is similar to convective flow in enclosed spaces subject to a lateral temperature gradient. As the settling particles migrate to one side of the inclined tube, the resulting density variation creates a pressure variation across the tube with a pressure gradient component that induces convective flow. The Boycott effect is therefore observed in inclined wells. If the settling of particles progresses far enough, or settling behaviour is not adequately taken into account, it can cause severe operational problems such as well instability, well kick, stuck-pipe, loss of circulation, problems with running a casing or liner and loss of the fluid column weight (a drop in hydrostatic pressure) which can lead to unwanted gas or oil influx into the well which, in severe cases, can cause a blowout.        
It is therefore particularly desirable to understand the drilling fluid quality during wellbore operations, so that operators can respond appropriately, for example to adjust procedures to reduce periods of no circulation or to change the composition of the drilling fluid. During the fluid design phase, typically performed in a laboratory, it is also essential to understand and quantify a degree to which various fluids are prone to sag in order to provide the most optimum fluid solution prior to field application.
Measurements may therefore be performed on the drilling fluid as it is pumped into or exits the well bore, providing an indication of whether the drilling fluid being used is suitable. In particular, “condition monitoring” is applied during drilling operations whereby repeated tests are performed on the drilling fluid to define a drilling fluid signature characterising the instantaneous state of the drilling fluid and/or of the well bore. A signature may be defined by a combination of numerous types of measurement performed on the drilling fluid. If the signature changes, it is an indication that the drilling fluid or wellbore state has changed. Knowing the current settling performance of a drilling fluid is an important indicator to take into account of and a key in avoiding operational problems particularly when the drilling fluid stands still over a period of time.
However, there is currently a lack of simple but reliable measurement for determining particle settling properties in drilling fluid. In particular, there is a lack of a good technique that is suitable for use in the field.
Note that, while the above description refers to the desire to keep a solid phase in suspension in a fluid, other applications may require a high degree of settling. For example, it may be required to use settling to segregate a solid phase from a liquid phase. In this case, it is also important to measure the settling properties of solid particles in a liquid, or settling properties of different liquid phases in an emulsion.
At present, there is often used a static sag test based on density measurements to provide an indication of settling behaviour. This technique involves taking a sample of drilling fluid, and then letting the particles settle within the fluid. Then, a discrete measurement of the density of an upper part and lower part of the sample mixture is made. The density measurements are combined for example as a ratio to give a numerical value or “sag factor” which is used as an indicator of particle sag. A measurement of density is made at the upper and lower parts once the settling has occurred. No information is provided on how settling progresses and the accuracy of results using this approach can be relatively poor.
Alternatively, dynamic measurement techniques may be used which take into account shear effects. For example, direct weight measurements can be made which take into account rotation and shear rate effects of particles, temperature, vibrations and fluid compositional measurements. To the knowledge of the inventors only a single example of direct weight measurement equipment is available for use currently, set up in an onshore laboratory. It is not suited to field use. Sag flow loop tests can be performed involving putting fluid to be measured into motion in a flow loop taking into account wellbore angle effects. This requires large and relatively complex measurement equipment.
Nuclear Magnetic Resonance (NMR) and viscometer testing are other potential methods.
Acoustic measurement techniques have also been proposed for characterising settling in fluid mixtures. Developments of these acoustic techniques are typically focused upon perfecting measurements transverse to the settling direction. Thus, transmission and detection of an acoustic signal occurs on a path along which the particle volume fraction or distribution of particles is substantially uniform. Upon settling under the influence of gravity, particles tend to move through the mixture in or parallel to the direction of gravity, substantially vertically, and then settle forming a layer of particles in the mixture. Existing techniques may use horizontal detection arrangements, across the settling direction, to target a region where the particle volume fraction is uniform.
The document US2004/00182138 describes a system for determining properties of settling suspensions. It refers to use of a supplementary transducer which operates in a vertical direction. The transducer is limited to detecting echo effects from particles in the suspension.